Molecular Designed Dispersion Research Articles

Catalytic activity of cobalt grafted on ordered mesoporous silica materials in N2O decomposition and CO oxidation

Three different ordered mesoporous silica materials, such as MCM-41, Al incorporated into the silica framework MCM-41 (mass ratio Si/Al=9) and SBA-15, were prepared. Furthermore, aluminum was grafted on the surface of MCM-41 (mass ratio Si/Al=16) and SBA-15 (mass ratio Si/Al=23) by the molecular designed dispersion (MDD) method. In a next step, cobalt (Co=10–12wt%), as an active metal for redox reactions, was introduced by the MDD technique. The prepared catalysts were characterized by atomic absorption spectroscopy, microwave plasma-atomic emission spectroscopy, N2 physisorption, thermogravimetric analysis, X-ray diffraction, diffuse reflectance UV–vis spectroscopy, infrared and Raman spectroscopies, transmission electron microscopy, temperature programmed reduction by hydrogen and their catalytic properties were evaluated for two model reactions: N2O decomposition and CO oxidation. All prepared catalysts showed poor activity in the reaction of N2O decomposition and the use of reducing agent (carbon monoxide) improved it only little. Contrary to this, all catalysts were significantly more active for CO oxidation by oxygen. The highest activity was reached over the SBA+Co catalyst which is proposed to be due to the smallest size of tetrahedral Co(II) species on SBA-15 silica support.

Comparison between a Water-Based and a Solvent-Based Impregnation Method towards Dispersed CuO/SBA-15 Catalysts: Texture, Structure and Catalytic Performance in Automotive Exhaust Gas Abatement

Supported copper oxide nanoparticles are a potential candidate for replacing the rare and expensive precious metals within the automotive three-way catalyst. However, a well-designed dispersion method is necessary to allow a stable high loading of active material, compensating its lower intrinsic activity and stability. In this work, a CuO-loaded SBA-15 catalyst has been manufactured by two methods. The ammonia-driven deposition precipitation (ADP) and the molecular designed dispersion (MDD) methods are both considered as efficient deposition methods to provide well-dispersed copper oxide-based catalysts. Their morphology, copper dispersion and the chemical state of copper were characterized and compared. Due to the differences in the synthesis approach, a difference in the obtained copper oxide phases has been observed, leading to a distinct behavior in the catalytic performance. The structure-activity correlation of both catalysts has also been revealed for automotive exhaust gas abatement. Results demonstrate that various copper species can be formed depending on the precursor–support interaction, affecting selectivity and conversion during the catalytic reaction.

SBA-15 mesoporous silica modified with rhodium by MDD method and its catalytic role for N2O decomposition reaction

SBA-15 mesoporous silicas modified with rhodium were studied as catalysts for the N2O decomposition reaction. Rhodium was deposited on SBA-15 by the Molecular Designed Dispersion (MDD) method using Rh(acac)3 as a precursor of active phase. The same method was used for the deposition of Cu, Fe, Al and Ti. The SBA-15 support modified with metals were characterized with respect to metal loading (EPMA), structure (XRD), texture (BET), morphology (SEM), Rh dispersion (oxygen chemisorption), surface acidity (pyridine adsorption) and chemical nature of introduced copper and iron species (UV–vis-DRS). The rhodium-containing SBA-15 samples were found to be active catalysts for the N2O decomposition reaction. Deposition of Al on the Rh-loaded catalyst increased its activity. An opposite effect was observed for the samples modified with Cu and Fe.

ChemInform Abstract: Molecular Dispersion of Metal Complexes Within Zeolitic Solids: An Alternative Way to Prepare Supported MOx Catalysts

Molecular Designed Dispersion (MDD) of metal complexes on a highly porous support is a novel synthesis method to prepare high quality heterogeneous catalysts. The process basically consists of two steps: (1) the anchoring of the complex onto the support in a controlled way and (2) the mild oxidation of the grafted complex towards catalytically active metal-oxide surface structures. This article presents two typical case studies: (1) the incorporation of cationic Cu-complexes in a H-ZSM-5 zeolite from the liquid phase and (2) the gas phase modification of pure silica MCM-48, using the VO(acac)2 complex. In both cases, superior catalysts are obtained. Detailed chemical, physical and catalytical analyses of these catalysts are discussed in the text. Comparison is made with analogous catalysts, prepared by conventional methods.

SBA-15 mesoporous silica modified with metal oxides by MDD method in the role of DeNO x catalysts

SBA-15 mesoporous silica was modified with metal (Al, Ti, Cu, Fe) oxides by the molecular designed dispersion (MDD) method using acetylacetonate complexes of metals as precursors of the catalytically active components. The modified mesoporous silicas were characterized with respect to texture (BET), composition (EPMA), coordination and aggregation of transition metal species (UV–vis-DRS), reducibility of the deposited transition metals (TPRed) and surface acidity (FT-IR). Deposition of aluminium and titanium species on the SBA-15 surface significantly increased its acidity, mainly by generation of strong Lewis acid sites. Copper and iron deposited on the surface of pure SBA-15 were present nearly exclusively in the form of mononuclear cations. Deposition of Fe or Cu on the SBA-15 supports modified with alumina or titania resulted in a formation of significant amounts of oligomeric metal oxide clusters. The SBA-15 based samples have been found to be active and selective catalysts of the DeNO x process. The modification of the silica surface with titanium or aluminium prior to the deposition of iron or copper significantly improved the activity of the SBA-15 based catalysts.

VO x supported SBA-15 catalysts for the oxidative dehydrogenation of ethylbenzene to styrene in the presence of N 2O

The molecular designed dispersion (MDD) method was used for the modification of SBA-15 with vanadium oxide. Vanadyl acetylacetonate was grafted on the support surface at different VO(acac) 2 concentrations in a toluene solution, and then converted into the VO x /SBA-15 catalysts at elevated temperatures. The thermal transformation of surface species was studied by FTIR-PAS spectroscopy. The obtained materials were characterized with respect to their texture (BET), metal dispersion (FT-Raman and UV–vis-DR) and reducibility (H 2-TPR). It was found that only isolated V 5+ species were present in the samples with a low V loading. The catalyst with the highest V content showed the presence of polymeric V 5+ species, which appeared to be easier reducible compared to the isolated form of V. The VO x /SBA-15 samples were tested as catalysts in the oxidative dehydrogenation of ethylbenzene to styrene in the presence of N 2O. An increase in the catalytic activity with raising the V content was observed. It was however prove that monomeric V 5+ species were considerably more active and selective in the styrene formation than oligomeric ones.

Effect of the preparation technique on the catalytic properties of mesoporous V-HMS for the oxidation of toluene

Various mesoporous catalysts with vanadium loadings between 0.5 and 6 V wt.% and surface areas around 1300 m 2/g were synthesized using the isomorphous substitution (IS) and molecular designed dispersion (MDD) techniques. Their catalytic properties were tested using toluene as a model VOC in a fixed bed reactor at temperatures between 300 and 550 °C. It was found that during the oxidation of toluene, over V-HMS synthesized via IS, conversion of toluene mainly results in carbon oxides, benzene, benzaldehyde and water. Total conversion is greatly improved when the vanadium content is increased from around 1.5 to 3.0 wt.%, but an increase in the textural porosity ( V TEX/ V MESO) from 0.3 to 0.6 had no discernable effect on the conversion. This can be explained by the fact that a V TEX/ V MESO as low as 0.3 is sufficient to facilitate the access of toluene into the framework confined mesopores without any molecular transport limitations. However, when using V-HMS synthesized by MDD, conversion of toluene is greatly improved when the V TEX/ V MESO ratio is increased from 0.1 to 0.6. This is because the diffusion limitations are minimized by this increase. V-HMS synthesized via MDD does not exhibit selectivity to benzaldehyde, favoring total oxidation to CO and CO 2. This different oxidation mechanism can be explained in terms of location, accessibility and number of active species on the surface of the HMS support.

Catalytic Activity of MCM-48-, SBA-15-, MCF-, and MSU-Type Mesoporous Silicas Modified with Fe3+ Species in the Oxidative Dehydrogenation of Ethylbenzene in the Presence of N2O

MCM-48, SBA-15, MCF, and MSU mesoporous silicas were used as supports for a deposition of Fe oxide species. Iron was introduced using two different methods: the wetness impregnation and the molecular designed dispersion (MDD). The obtained catalysts were characterized with respect to their textural parameters (BET), chemical composition (electron microprobe analysis), and reducibility (TPR). The coordination environment of Fe was determined using EPR and UV-vis/DRS. The samples were tested as catalysts in the oxidative dehydrogenation of ethylbenzene to styrene in the presence of N(2)O. An influence of Fe dispersion and reducibility on the catalytic activity was discussed. Isolated Fe(3+) species appeared to be more selective in the styrene formation, whereas iron oxide clusters showed a higher selectivity in total oxidation of aromatic hydrocarbons. The reaction system was well described by the Mars- van Krevellen mechanism.

Catalytic performance of various mesoporous silicas modified with copper or iron oxides introduced by different ways in the selective reduction of NO by ammonia

Mesoporous silicas (MCM-48, SBA-15, MCF), reflecting various porous structures, were modified with copper and iron oxides by two different methods. For a first series of the samples the molecular designed dispersion (MDD) method using acetylacetonate complexes of copper and iron was applied for the deposition of transition metal oxides on the silica supports. A second series of the catalysts was obtained by the incipient impregnation technique using aqueous solutions of the suitable metal nitrates. The modified materials were characterized with respect to the texture (BET), composition (electron microprobe analysis), coordination of the transition metals (UV–vis–DRS) and surface acidity (NH3-TPD, FTIR). The mesoporous silica supports modified with transition metal oxides were tested as catalysts of the selective reduction of NO with ammonia. The catalytic performance of the studied samples depended on the method used for the deposition of transition metal oxide as well as the kind of mesoporous silica used as a catalytic support. In general, the Cu-containing mesoporous samples effectively operated at lower temperatures than silicas modified with iron. The samples obtained by the MDD method have been found to be more active and selective compared to the analogous samples prepared by the impregnation technique. An introduction of water vapor into the reaction mixture only slightly decreased the NO conversion and selectivity towards N2 over the MCF mesoporous silica modified with copper or iron oxide.

Diffusion effects in SBA-15 and its plugged analogous by a deposition of metal–acetylacetonate complexes

Plugged hexagonal templated silica (PHTS) is a porous material analogous to SBA-15, which possesses extra microporous amorphous silica nanoparticles (plugs) within the mesopores. The influence of these nanoparticles in the mesopores of PHTS on the deposition of large metal–acetylacetonate complexes was investigated. TiO(acac) 2 and VO(acac) 2 were deposited on pure SBA-15 and PHTS using the liquid phase molecular designed dispersion (MDD) method. Thermolysis of the adsorbed complexes creates isolated metal oxide species in the channels of the support material. Chemical analysis, N 2 sorption measurements, FTIR-PAS, UV–VIS diffuse reflectance and FT-Raman were used to evaluate the surface characteristics, surface coverage and final metal oxide loading. Information on important diffusion differences between SBA-15 and its plugged analogous was obtained. The size of the metal–acetylacetonate complex and the temperature during the deposition are important parameters for diffusion in PHTS and seem to be controlling the final properties of the deposited metal oxide layer and loading. Additionally, the influences of temperature variations during the synthesis were examined.

Characterization of Vanadium and Titanium Oxide Supported SBA-15

Supported vanadium and titanium oxide catalysts were prepared by adsorption and subsequent calcination of the vanadyl and titanyl acetylacetonate complexes, respectively, on mesoporous SBA-15 by the molecular designed dispersion (MDD) method. Liquid and gas phase depositions at different temperatures were carried out with vanadyl acetylacetonate, and the different results together with those of titanyl acetylacetonate in the liquid phase deposition were discussed. The bonding mechanism, the influence of the metal interaction with the support material, and differences due to the way of deposition and the temperature were investigated by TGA, chemical analysis, FTIR, and Raman spectroscopy. Elevated dissolving temperatures in the liquid phase led to higher final loadings on the SBA-15 without the formation of clusters, even at high loadings. The decomposition of the anchored vanadium and titanium complexes, their thermal stability, and the conversion to the covalently bound VO(x) and TiO(x) species on SBA-15 were studied and investigated by in situ transmission IR spectroscopy. In general, the titanium complex is more reactive than the vanadium complex toward the surface of SBA-15 and has a higher thermal stability. The MDD method of the VO(acac)2 and TiO(acac)2 enables to create a dispersed surface of supported VO(x) and TiO(x), respectively. The structure configurations of VO(x) and TiO(x) oxide catalysts obtained at different metal loadings were studied by Raman spectroscopy. Pore size distributions, XRD, and N2 sorption confirmed the structural stability of these materials after grafting. VO(x)/SBA-15 and TiO(x)/SBA-15 samples, with different metal loadings, were also catalytically tested for the selective catalytic reduction (SCR) of NO with ammonia.

Characterisation and reactivity of vanadia–titania supported SBA-15 in the SCR of NO with ammonia

TiOx, VOx and TiOx–VOx oxides were highly dispersed at different metal loadings on the mesoporous silica SBA-15 by the molecular designed dispersion (MDD) method. A combination of different techniques (N2-sorption, FT-Raman, TPR, UV–vis-DR) was used to verify the nature of the vanadia and/or titania species on the surface of the mesoporous support. Temperature-programmed desorption (NH3-TPD) was applied to characterise the surface acidity of these catalysts. Vanadia and titania were found to be present as isolated species, even at high metal oxide concentrations. The supported catalysts were tested in the selective catalytic reduction (SCR) of NO with ammonia. Among the studied samples, the highest activity was found for TiOx–VOx mixed oxides on SBA-15. The catalytic activity results were discussed in terms of loading and metal dispersion on the support material.

Synthesis, characterization and hydrodesulphurisation activity of CoMo/γ-Al 2O 3 catalyst prepared through molecular designed dispersion method

CoMo/γ-Al 2O 3 catalysts for hydrodesulphurisation activity were prepared by making use of the molecular designed dispersion (MDD) method. Molybdenum and cobalt pyrrolidine- N-carbodithioate (Pydtc) complexes were used for the incorporation of metals on the support. The catalysts were characterized by elemental analysis, low temperature oxygen chemisorption, temperature programmed reduction (TPR) and laser Raman spectroscopy. The hydrodesulphurisation activity of all the catalysts were carried out and results were compared with those of the catalysts prepared through the conventional method. Higher molybdenum dispersion, smaller molybdenum clusters, lower reduction temperature of catalyst and better hydrodesulphurisation activity were observed for the catalysts prepared through the MDD method.

Molecular Dispersion of Metal Complexes within Zeolitic Solids: An Alternative Way to Prepare Supported MOx Catalysts

Molecular Designed Dispersion (MDD) of metal complexes on a highly porous support is a novel synthesis method to prepare high quality heterogeneous catalysts. The process basically consists of two steps: (1) the anchoring of the complex onto the support in a controlled way and (2) the mild oxidation of the grafted complex towards catalytically active metal-oxide surface structures. This article presents two typical case studies: (1) the incorporation of cationic Cu-complexes in a H-ZSM-5 zeolite from the liquid phase and (2) the gas phase modification of pure silica MCM-48, using the VO(acac)2 complex. In both cases, superior catalysts are obtained. Detailed chemical, physical and catalytical analyses of these catalysts are discussed in the text. Comparison is made with analogous catalysts, prepared by conventional methods.